Process for assisting in the removal of scale from alloy steel



e 2,863,792 Egg I Patented Dec. 9, 1958 PROCESS FOR ASSE'HNG IN THE REMOVAL F SCALE FROM ALLUY STEEL Hoyt H. Todd, Whittier, Califi, assignor to Superweld Corp, North Hollywood, Calif., a corporation of California No Drawing. Application August 23, 1957 Serial No. 680,0tl6

9 Claims. (Cl. 148-614) This invention relates to a process for assisting in the removal of scale from alloy steels, and more particularly for the removal of scale from steels of compositions containing over a certain percentage of various elements such as chromium, tungsten, vanadium, molybdenum, etc. The present application is a continuation in part of my copending application of the same title, Serial No. 117,503, filed September 23, 1949.

The objects of the invention are:

First, to provide a process which is very economical because the particular chemicals used therein are cheap and easily obtainable;

Second, to provide a process of this class which may be carried on in conventional furnaces ordinarily employed for normalizing stainless steel or other similar alloy metals to which the process is applicable;

Third, to provide a process of this class in which all of the reactions which form a soluble scale on the material take place in the furnace used for annealing or other heat treatment of the material;

Fourth, to provide a process of this class in which the material being treated is not attacked or injured in any way by corrosion or otherwise inasmuch as the chemicals used only enter into the reaction which forms the soluble scale on the material;

Fifth, to provide a process of this class in which the production of the soluble scale takes place at a higher temperature than in conventional molten bath furnaces whereby the formation of the soluble scale is more complete;

Sixth, to provide a process of this class in which no hazardous, corrosive, poisonous, or hot chemicals are necessary which would render the process very safe compared to other processes now in use;

Seventh, to provide a process of this class in which a liquid composition in the form of an aqueous solution is applied to the surface of the material previous to the introduction thereof into a furnace, which causes a soluble scale to be formed on the material, so that it may be readily removed by a conventional acid pickling bath, rendering the parts bright and clean;

Eighth, to provide a process of this class in which the application of the liquid composition preliminary to the introduction of the parts into the furnace may be accomplished by dipping, spraying or brushing the liquid composition on the surface of the material or parts being processed;

Ninth, to provide a process of this class which may be used in connection with conventional processing equipment without adding to or detracting from such equipment, which is normally used in processing stainless steel or the like in the conventional manner;

Tenth, to provide a process of this class in which no sulphuric acid rotting is necessary before descaling the parts being processed;

Eleventh, to provide a process of this class which is very simple and economical to operate, efiicient in its action, and which is very easy to follow.

With these and other objects in view, as will appear hereinafter, my invention consists of a certain novel process as will be hereinafter described in detail and particularly set forth in the appended claims.

Heretofore, it has been difficult to remove the dark colored scale from stainless steel or other alloys containing over a total of 10% of chromium, tungsten, va= nadium or molybdenum. In the conventional process, parts formed of such materials when being normalized in a furnace accumulate a hard surface scale which is very difficult to remove even in the acid baths composed of nitric acid, hydrofluoric acid and the preliminary baths of hydrochloric acid. In my process the parts are subjected to a bath or spray coating preliminary to the entrance of the part into the furnace in which they are normalized. In my process, the bath or spray coating for the parts may be a liquid composition comprising any one of several salts or hydroxides of the alkali metals. Examples of the alkali metals referred to are as follows: Na PO .l2H O; NaOI-I; KOH; NANO and other similar materials.

It is to be noted that the salts or hydroxides of the foregoing elements are of such a composition that they form, upon decomposition at temperatures of approximately 500 to 1200 C. in the furnace, a basic salt or oxide capable of combining at high temperature with the acidic oxide of the alloying elements, forming an alkali metal salt, soluble in the usual pickling solution consisting of various acids such as hydrochloric acid, nitric acid or a mixture thereof with hydrofluoric acid.

An example of a liquid composition that may be used having an alkali metal salt content is substantially as follows in terms of percentage of weight of the total liquid composition: 5 to 10% trisodium phosphate; 10% sodium carbonate; A% wetting agent or conventional detergent; 10% isopropyl alcohol; and 5% sodium nitrate.

An advantage of including sodium nitrate is that it is a deliquescent or hygroscopic material which maintains the moisture in the applied coating. The wetting agent which, for example, may be Nacconal, is provided to reduce surface tension on the parts and to provide an even wet coating of the liquid composition as sprayed on the parts preliminary to the heat treatment. The alcohol tends to clean or degrease the parts in the event any small amounts of oil or grease remain on the parts when subjected to the bath or the spray which is arranged to provide an even coating on the parts.

In actual practice, the stainless steel or other material being processed is preferably degreased and then subjected to a spray or bath of the liquid composition hereinbefore set forth, such liquid composition including water or other liquid vehicle for the elements of the composition so that the parts may either be immersed in the liquid composition or the liquid composition may be sprayed on the parts. After the parts have been thoroughly covered with the liquid composition of the invention including at least one of the hydroxides or at least one of the salts, the parts are then subjected to the interior of the furnace in which good circulation is believed to be helpful in obtaining uniform results.

'During the subjection of the parts to high temperature for normalizing thereof, the decomposition of the hydroxide and/ or salt content of the liquid composition produces an alkaline reacting substance to combine with the acidic oxides of the alloying elements to form a coating which is readily soluble in the usual pickling solution employed for the descaling of stainless steel. After the parts have been removed from the furnace they are then subjected to the pickling solution which may con sist of hydrochloric acid, nitric acid or hydrofluoric acid mixed with nitric acid or other suitable material.

It is to be noted that various metal salts may be used in the pro-heat treatment liquid composition and that the term alkali metal salts used herein includes alkaline earth metal salts. It is obvious that any suitable alkali metal salts having low surface tension when molten may be used in the foregoing process. The various alkali metal salts that decompose at high temperature to form alkaline reacting substances may be readily selected by reference to a chemical handbook.

My process is concisely as follows: The parts are first coated with the salt or hydroxide of an alkali metal by spraying or by immersion in the cold liquid composition, then the parts are subjected to high temperature forming an alkali metal salt thereon during the normalizing of the parts, and then the parts are subjected to a conventional pickling solution in which the alkaline metal salt is readily removed. The pickling solution leaves the parts clean and bright and does not unduly corrode the parts because of the short term required for the pickling step.

The general nature of the numerous reactions involved in the process may be appreciated by considering the previously mentioned example in which the coating comprises five materials, namely, trisodium phosphate, sodium carbonate, wetting agent, isopropyl alcohol, and sodium nitrate.

Suppose the metal that is coated is a chromium-steel alloy. The sodium carbonate decomposes at high temperature to release CO thus provides an oxidizing environment, thus:

The sodium nitrate also decomposes to release oxygen and thus provides the oxidizing environment, thus:

In the oxidizing environment provided by C the following reactions occur:

ZNa CrO, is readily soluble in an acid pickling bath. In the oxidizing environment provided by the 0 that is released by the decomposition of the sodium nitrate, the presence of sodium carbonate results in the following reaction:

If the coating includes a hydroxide, the oxidizing atmospherc causes Cr O to be converted thus:

The statement made heretofore that good circulation in the interior of the furnace is believed to be helpful can be appreciated in view of the fact that the atmosphere provides oxygen to form at least a part of the oxidizing environment. It is obvious that a heat treating step such as a conventional normalizing step inherently provides an oxidizing environment since the metal scale that constitutes the problem solved by this invention is produced by oxidation during the normalizing step. The present invention uses the same oxidizing environment in the normalizing furnace to produce either a higher oxide of the alloying metal or a higher oxygencontaining salt of the alloying metal, either of which is soluble in an acid pickling bath.

The coating bath may comprise a single ingredient and depend on the atmoshpere in the normalizing furnace to provide the oxidizing environment or a single ingredient may he used that will inherently produce its own oxidizing environment. Thus, if the single ingredient is NaOH. the oxidizing environment provided by the atmosphere in the normalizing furnace will be utilized:

SHZtOH-l-30 +2C1' O 4Na CrO -l4H 0 If the single ingredient comprises Na CO this ingredient will provide its own oxidizing environment independently of the atmosphere:

A coating bath of two ingredients may comprise Na CO and NaNO in which the NaNO furnishes the oxidizing environment for the salt:

A coating bath of three ingredients, Na CO NaNO and NaOH may be used with the first two ingredients providing the oxidizing environment for the hydroxide:

Though I have described a particular combination of elements, I do not wish to be limited to this particular combination of elements nor to the particular proportions thereof but desire to include in the scope of my invention the process and the elements employed in connection therewith substantially as set forth in the appended claims. Having thus described my invention what I claim is new and desire to secure by Letters Patent is:

1.ln the processing of metals containing at least one steel-alloying element that forms an acidic oxide, which processing includes a step of heat treating the metal in an oxidizing atmosphere, the improvement for preventing the formation of dense, firmly adherent oxides on the surface of the metal during the heat treating step, which improvement consists in coating the metal prior to the heat treating step with an oxygen-containing metal compound which forms an alkaline reacting substance on thermal decomposition, said compound being selected from a group consisting of alkali metal hydroxides, alkaline earth hydroxides and oxygen-containing alkali metal salts, whereby the heat treating step in the oxidizing environment causes the acidic oxide of the steelalloying metal to be converted into a compound capable of solution in an acid pickling solution and then pickling the thus processed metal in an aqueous inorganic acid pickling solution after the heat treating step.

2. The improvement as set forth in claim 1, in which the oxygencontaining metal compound of the coating is combined with a wetting agent.

3. The improvement as set forth in claim 1 in which the oxygen-containing metal compound of the coating is combined with a detergent.

4. The improvement as set forth in claim 1, in which the oxygencontaining metal compound of the coating is combined with an alcohol.

5. The improvement as set forth in claim 1, in which the coating applied to the metal includes an alkali metal inorganic hygroscopic salt to maintain moisture in the coating.

6. In the processing of metals containing at least one steel-alloying element that forms an acidic oxide, which processing includes a step of heat treating the metal in an oxidizing atmosphere, the improvement for preventing the formation of dense, firmly adherent oxides on the surface of the metal during the heat treating step, which improvement consists in coating the metal prior to the heat treating step with a composition including at least one oxygen-containing alkali metal salt, a wetting agent and an alcohol, which composition forms an alkaline reacting substance on thermal decomposition, whereby the heat treating step in the oxidizing environment causes the acidic oxide of the steel-alloying metal to be converted into a compound capable of solution in an acid pickling solution and then pickling the thus processed metal in an aqueous inorganic acid pickling solution after the heat treating step.

7. The improvement as set forth in claim 6, in which said composition includes a hygroscopic material.

8. In the processing of metals containing at least one steel-alloying metal that forms an acidic oxide, which processing includes a step of heat treating the metal in an oxidizing atmosphere, the improvement for preventing the formation of dense, firmly adherent oxides on the surface of the metal, which improvement consists in coating the metal prior to the heat treating step with an oxygen-containing metal compound which forms an alkaline reacting substance on thermal decomposition, said compound being selected from a group consisting of alkali metal hydroxides, alkaline earth hydroxides and an alkali metal salt from the group consisting of trisodium phosphate and sodium carbonate, whereby the heat treating step results in a coating that is readily removable by an acid pickling solution and then pickling the thus processed metal in an aqueous inorganic acid pickling solution after the heat treating step.

9. In the processing of metals containing at least one steel-alloying metal that forms an acidic oxide, which processing includes a step of heat treating the metal in an oxidizing environment, the improvement for preventing the formation of dense, firmly adherent oxides on the surface of the metal, which improvement consists of coating the metal prior to the heat treating step with an oxygen-containing metal compound which forms an alkaline reacting substance on thermal decomposition, said compound'being selected from a group consisting of alkali metal hydroxides, alkaline earth hydroxides and an alkali metal salt from the group consisting of trisodium phosphate and sodium carbonate, said compound being effective, in the oxidizing environment of the heat treating step, to cause the acidic oxide of the steel-alloying metal to be converted into a compound capable of solution in an acid pickling solution and then pickling the thus processed metal in an aqueous inorganic acid pickling solution after the heat treating step.

References Cited in the file of this patent UNITED STATES PATENTS 2,065,947 Nauth Dec. 29, 1936 2,150,777 Morrill Mar. 14, 1939 2,301,983 Tanner Nov. 17, 1942 

1. IN THE PROCESSING OF METALS CONTAINING AT LEAST ONE STEEL-ALLOYING THAT FORMS AN ACIDIC OXIDE, WHICH PROCESSING INCLUDES A STEP OF HEAT TREATING THE METAL IN AN OXIDIZING ATMOSPHERE, THE IMPROVEMENT FOR PREVENTING THE FORMATION OF DENSE, FIRMLY ADHERENT OXIDES ON THE SURFACE OF THE METAL DURING THE HEAT TREATING STEP, WHICH IMPROVEMENT CONSISTS IN COATING THE METAL PRIOR TO THE HEAT TREATING STEP WITH AN OXYGEN-CONTAINING METAL COMPOUND WHICH FORMS AN ALKALINE REACTING SUBSTANCE ON THERMAL DECOMPOSITION, SAID COMPOUND BEING SELECTED FROM A GROUP CONSISTING OF ALKALI METAL HYDROXIDES, ALKALINE EARTH HYDROXIDES AND OXYGEN-CONTAINING ALKALI METAL SALTS, WHEREBY THE HEAT TREATING STEP IN THE OXIDIZING ENVIRONMENT CAUSES THE ACIDIC OXIDE OF THE STEELALLOYING METAL TO BE CONVERTED INTO A COMPOUND CAPABLE OF SOLUTION IN AN ACID PICKLING SOLUTION AND THEN PICKLING THE THUS PROCESSED METAL IN AN AQUEOUS INORGANIC ACID PICKLING SOLUTION AFTER THE HEAT TREATING STEP. 